New proteomic technologies are urgently needed that can meet the demand of emerging large-scale biological studies. We have developed a novel microarray analysis platform, which enables screening of large bead libraries using MALDI TOF mass spectrometry imaging. While biological tissue imaging by mass spectrometry is now a well-established technique, MALDI TOF MSI has not been used to perform scanning of biological microarrays. Our MSBeadArray technology enables transfer of selected analytes from beads onto the surface of a specially designed microarray slide under conditions, which maintain spatial separation of analytes from individual microbeads while preserving co-localization of analytes extracted from the same bead. The unique features of this platform include MS analysis of bead libraries containing up to 500,000 samples on a single microarray slide, dual fluorescence - MSI readout and the option to perform label-free sample detection that eliminates the need for fluorescent labels and detection antibodies. Here we propose to apply the MSBeadArray technology to the development of libraries of bead-conjugated peptides, which will be measured by mass spectrometry in a microarray format. Peptide bead arrays with the mass spectrometry readout will have significant advantage over both planar peptide microarrays with positional encoding and suspension bead arrays with the fluorescent encoding (e.g., the Luminex platform). Because mass spectrometry can detect multiple peptide species in a single measurement, the peptide purity, presence of additional sequences and even relative amounts of each peptide per bead are measured during the microarray scan. Peptide manufacturers routinely employ MALDI TOF MS analysis to perform QC of the synthesized peptides that are used to produce microarrays. However the fabricated microarrays are not analyzed by MS, therefore the quality of peptide probes, immobilized on the microarray slides or beads, is not known. In contrast, our technology performs QC on every member of the bead microarray after the microarray reaction is completed. Variability of bead assays will be substantially reduced because of the probe quality data provided by the MSBeadArray platform.